1. Field of the Invention
The present invention relates to a holographic recording/reproducing apparatus, and more particularly, to a holographic recording/reproducing apparatus having an aperture of variable size.
2. Description of the Related Art
Hologram technology is being used to record/reproduce an optical signal in/from a stereoscopic image by recording an interference pattern between a recording signal beam, which carries a signal, and a recording reference beam, which makes a predetermined angle with the recording signal beam. Optical storage technology for recording and reproducing digital data using holographic principles has recently been highlighted, and will be referred to hereinafter as holographic information recording and reproducing technology. The holographic information recording and reproducing technology allows recording and reproducing of data or information in units of pages, by which a plurality of digital data are simultaneously recorded/reproduced in a shape of a two-dimensional image. Thus, an ultra-high speed recording/reproducing system can be implemented thereby. In addition, the holographic information recording and reproducing technology can even separate and read information which is spatially overlapped and stored in a holographic recording medium by using a proper multiplexing technique. Thus, data or information of several pages can be recorded in an overlapping manner and be reproduced from the same region of the holographic recording medium.
FIG. 1A schematically illustrates a principle of recording information using holographic information recording and reproducing technology in a typical holographic information recording/reproducing apparatus. As illustrated in FIG. 1A, a beam splitter 2 splits a laser beam 1 into a reference beam (or a recording reference beam) 6 and a signal beam (or a recording signal beam) 5. The signal beam 5 passes through a spatial light modulator (SLM) 4, is modulated into a two-dimensional signal pattern, and is then incident on a holographic recording medium M. An aperture (or an aperture device) a may be disposed on the optical path of the signal beam 5 so that only a desired signal beam can be incident on the holographic recording medium M. Meanwhile, the reference beam 6 is reflected by a mirror 3 and is incident on the holographic recording medium M at a predetermined angle, causing the reference beam 6 to interfere with the signal beam 5. An interference pattern formed in this way is recorded in the holographic recording medium M.
FIG. 1B illustrates a principle of reproducing recorded information using a recorded hologram in the typical holographic information recording/reproducing apparatus. When information is to be reproduced, a light source 8 is used to emit a beam of light 6′ to be irradiated on the holographic recording medium M. The light 6′ is a reproducing reference beam of the same wavelength as the reference beam 6 that is used when information is recorded. The reference beam 6′ should be incident on the holographic recording medium M at the same angle of incidence as when the information is recorded. Then, a signal beam (a reproducing signal beam) 5′ having a two-dimensional signal pattern containing original data information is generated from the reference beam 6′ by diffraction via the holographic recording medium M. The signal beam 5′ is condensed by a lens 9, and is then detected by a two-dimensional photodetector 10, such as a charge coupled device (CCD). The aperture a is disposed on the optical path of the signal beam 5′ so that only a desired signal thereof can be detected by the photodetector 10 and other signal beams can be cut off.
The holographic recording/reproducing apparatus records a two-dimensional signal pattern in predetermined units in the holographic recording medium M. Generally, a two-dimensional region, which is a unit of recoding/reproducing information in/from the holographic recording medium M, is referred to as a book. For example, information of various pages can be simultaneously recorded on the holographic recording medium M in a multiplexing manner, such as angular multiplexing, in one book.
Meanwhile, to increase a recording density of the holographic recording medium M, it is important to narrow an interval between books in the holographic recording medium M. Additionally, an optical spot of the two-dimensional signal pattern, which is formed on the holographic recording medium M, should have an optimum size. To increase the recording capacity of the holographic recording medium, InPhase Technologies of Longmont, Colo. has suggested a method in which a polytopic aperture (or a polytopic aperture device) is used. The polytopic aperture has a size 1.1 times that of a Nyquist aperture (or a Nyquist aperture device), which has a minimum aperture size, so that an image frequency component of the spatial light modulator (SLM) may be transmitted without loss. The Nyquist aperture has a size equal to half of a size of the optical spot. The Nyquist aperture transmits all image frequency components, but deteriorates the quality of a signal. Accordingly, an appropriate aperture, a size of which is slightly greater (e.g., 1.1 times) than that of the Nyquist aperture, can be used such that the required signal quality may be obtained. Such an aperture is referred to as a polytopic aperture.